THALLOPHYTA: FUNGI 153 



FUNGI IMPERFECTI 



The Fungi Imperfect! constitute a large assemblage of forms that, 

 because of an incomplete knowledge of their life histories, cannot be 

 assigned to any of the three natural classes of true fungi: the Phycomy- 

 cetes, Ascomycetes, and Basidiomycetes. Generally the only known 

 method of reproduction is by conidia. Zygotes, ascospores, or basidio- 

 spores are unknown. In many cases the unknown stage has apparently 

 been lost from the life history. When a member of this artificial group is 

 found to possess any reproductive stage previously not reported, it is 

 transferred to its proper genus, family, order, and class. Meanwhile it is 

 placed in a "form genus." Many of the imperfect fungi cause important 

 plant diseases, such as potato scab, early blight of potato, flax wilt, and 

 various anthracnose and leaf-spot diseases. Practically all the fungi that 

 cause such human diseases as ringworm and athlete's foot are imperfect 

 fungi. 



LICHENES 



A lichen is a plant consisting of a unicellular alga and a fungus living 

 together in symbiotic relationship. This association, resulting in a body 

 having a distinctive form and structure, suggests a single plant rather than 

 a composite one. Lichens are commonly regarded as constituting an 

 autonomous group of thallophytes, the Lichenes, which are either made 

 coordinate with the Algae and Fungi, or included with the latter as a 

 distinct class. By those who consider lichens to be merely fungi parasitic 

 upon algae, they are sometimes broken up and distributed among the 

 fungous groups that they most closely resemble. 



Lichens are commonly seen growing on rocks, tree trunks, dead wood, 

 and on the ground. They are a widely distributed group of which about 

 400 genera and 15,000 species are known. A few of the largest genera are 

 Lecidia, Buellia, Lecanora, Parmelia, Physcia, Collema, Stida, Cladonia, 

 Ramalina, and Usnea. Lichens are mostly gray or grayish green, but 

 some are more conspicuously colored. Based on their external form, 

 three general types are recognized: (1) crustose Hchens, w^hich occur as 

 incrustations on rocks and bark; (2) foliose lichens, which are flat, 

 leaf -like, and only partially attached to the substratum; and (3) fridicose 

 hchens, branching forms that hang from trees or grow either erect or 

 prostrate on the ground (Fig. 125). 



The greater part of a Hchen is composed of a compact mass of tangled 

 fungous hyphae, among which are numerous algal cells, either scattered 

 irregularly or in a definite layer (Fig. 126). The body is usually differ- 

 entiated into a compact cortical region and a lower region of looser tex- 

 ture, in either of which the algal cells may occur. In some lichens the 



